There are known gear wheels which engage with a roller pinion. Such gearing systems are intended for use in applications for driving aircraft wheels by means of an electric motor which drives a roller pinion engaging with a gear wheel, which is fixed to the aircraft wheel. However, because of the deformation of the landing gear that carries the aircraft wheel, and the deformation of the aircraft wheel itself, the centre-to-centre spacing varies over a significant range, and this must be taken into account in order to avoid any premature wear, or even jamming. In particular, deformation may cause a reduction in the centre-to-centre spacing, and provision must be made to ensure that the rollers are not trapped in, or fail to contact, the bottom of the tooth space.